When it leaves the incinerator, the slag that is generated in garbage and waste incinerators contains harmful substances that preclude the use of such slag for building material; storing such slag in dumps also poses a threat to the environment. The content of harmful substances depends on the composition of the garbage or waste that is to be incinerated, and also on the degree of efficiency of the incineration process. On average, the content of harmful substances in such slag, which is generated in incinerators used for domestic garbage and waste can vary in the range from approximately 50 to 100 g per kg of slag. The harmful substances contained in the slag can be, in particular, chloride, lead, cadmium, zinc, iron, copper, and other contaminants. Appropriate analyses have been published, for example, in Schweizer Ingenieur und Architekt [Swiss Engineer and Architect], Vol. 19, 1983. When such slag is stored in dumps, the percolating ground water must be collected and disposed of, otherwise it constitutes a serious threat to the enviroment. Corresponding problems occur-when such slag is used for road and highway construction, for which reason, in such cases, the slag is often mixed with cement and so rendered impermeable to water. Since fewer and fewer dumps suitable for slag are available, the storage or dumping of such slag is causing major problems. The use of such slag in road construction is also being restricted to a small number of applications because of the formerly high contents of harmful substances.
The secondary treatment of slag from garbage incinerator processes is already known. The journal Umweltmagazine [Environment Magazine], Nov., 1985, pp. 30-32, describes a process used to minimize the quantity of slag produced. In this process, a so-called wet deslagger, within which the slag is cooled, is arranged beneath the furnace, i.e., at the end of the furnace grate. It is also noted that the content of heavy metals is also reduced by this wet deslagging. However, this reduction of the heavy-metal content is unsatisfactory, and considerable quantities of harmful substances remain in the slag. If it remains in the wet deslagger, the slag breaks down, and this causes additional difficulties during handling. The use of a wet deslagger has been known in other plants for a considerable time, wherein, in many instances, a water-filled slag channel is used. The cooled slag is moved to a slag bunker from this water-filled channel, where it is stored until finally dumped. Very high storage and transportation weights and, as a consequence of these, very high disposal costs result from the high water content in the slag.
The publication Stuttgarter Berichte zur Abfallwirtschaft [Stuttgart Reports on Waste Management], Berlin, 1984, pp. 221-246, asserts that the major part of water-soluble harmful substances is concentrated in fly dust, and it has been recommended by the federal environmental protection agencies that only dust-free slag be used, for example, in road construction. DE 3700608A1 describes a process and an apparatus for processing slag, in which, after the slag has been quenched in a water bath, the finc components with a grain size of up to 1 mm are removed. The remaining slag has a reduced content of harmful substances and can be used for highway and road construction. The known processes for cleaning the slag still do not satisfy the increasing demands made on them. The slag that has been cleaned still contains residual components of water-soluble harmful substances The chloride content can still amount to 50% of the original quantity, for example. The fact that the slag has to be stored after having been quenched in a water bath is also a disadvantage, for large storage capacities are still required. The drop water that is produced must also be disposed of, for it is contaminated. The harmful substances in the dust components are floated in the slag by the water bath and adhere to the pores after drying. The proposed drying to a residual moisture content of 1% requires considerable amounts of energy when large quantities of slag are involved. When dried to such an extent, the cleaned slag is not suitable for use as construction material and has to be rewetted in order to be used, for example, for highway and road construction.